1. Field of Technology
This invention relates to conveyance technologies for accurately conducting sheet conveyance in an image forming apparatus.
2. Description of Related Art
In the image forming apparatus, an image is formed at a prescribed position on a sheet by transferring the sheet to an image forming position, namely to a transfer position by synchronizing the conveyance with the image formation.
The image forming apparatus and an image reading apparatus are required to accurately control and convey an original document and a sheet as a recording medium in terms of conveyance timing, the position in the conveyance and width direction, and inclination. However, since there are various kinds and characteristics of the sheets to be conveyed, and further, there are various kinds of environments, such as temperature and humidity in the conveyance, it is unavoidable that conveyance misalignment occurs.
With respect to the means for decreasing the position misalignment in the conveyance direction (vertical misalignment) and the inclination, in general, a technique for hitting the front edge of the sheet to a registration roller and for forming a sheet loop in the upstream of the registration roller has been generally utilized
Further, technologies for correcting the conveyance misalignment have been developed.
For example, Japanese Patent Application Publication Open to Public Inspection No. S63-207670 (JPA63-207670) proposed a technique for detecting misalignment in the main scanning direction, namely the direction, which is perpendicular to the sheet conveyance direction, and correcting a write start poison in the main scanning direction based on the detected result in the print operation.
Japanese Patent Application Publication Open to Public Inspection No. H08-268610 (JPA08-268610) has proposed a technique of using a movable conveyance roller to be controlled its inclination to correct the lateral misalignment of a sheet based on the detected results of a sheet edge detection means.
Japanese Patent Application Publication Open to Public Inspection No. 2000-335010 (JPA2000-335010) has proposed a technique for determining a write position by detecting the sheet passage position in the main scanning direction (the direction, which is perpendicular to the sheet conveyance direction) as an absolute value based on the detected results by a sensor.
Japanese Patent Application Publication Open to Public Inspection No. 2000-35709 (JPA2000-35709) has proposed a technique for correcting slant sheet conveyance by revolving a pair of correction roller provided between two pairs of registration rollers around an axis, which is perpendicular to the conveyance direction, under the condition where the sheet is released from a nip of registration rollers and fastened by the correction rollers.
Japanese Patent Application Publication Open to Public Inspection No. H06-234441 (JPA06-234441) has proposed a registration apparatus for detecting a sheet inclination angle, swing a conveyance roller until the nip becomes parallel to the sheet front edge, nipping the sheet and further swing the conveyance roller so as to be perpendicular to the conveyance direction to convey the sheet when the sheet inclination angle is detected. Further, JPA06-234441 has proposed a technique for moving the conveyance roller in the thrust direction when lateral misalignment of the sheet exists.
In the high performance image forming apparatus, which is capable of forming a high quality image, such as a color image forming apparatus and a monochrome image forming apparatus used in a short-run printing field, those apparatuses are required to be accurate in the image position on the recording material. The required image position accuracy includes the accuracies of a vertical position, a lateral position and the inclination of the image.
On the other hand, in a high-speed apparatus, since the conveyance velocity of the recording medium is high, a minute timing shift causes a large misalignment of the image position. Further, when the recording material conveyance is not precisely conducted, inferior conveyance occurs, which causes image position misalignment and paper jam.
As described above, since the conditions for the conveyance of recording medium tend to become severe, the conventional conveyance technique becomes hard to conduct enough correction.
The technique for detecting conveyance misalignment and correcting the image writing position in the image writing operation, which was disclosed in JPA63-207670 and JPA2000-335010 cannot be used in the high-speed color image forming apparatus.
In the high-speed color image forming apparatus, a plurality of image forming sections is sequentially disposed along an image carrier. Since a plurality of monochromatic images is formed in the process, where the image carrier rotates through a revolution, it is inevitable that the distance from the first monochromatic writing position to the transfer poison of a multi-color image, onto which several monochromatic images are synthesized, becomes long. Thus, since write timing of the first monochromatic image becomes earlier than conveyance control timing of recording medium, the write position control based on the recording medium conveyance misalignment becomes impossible.
In the case when applying the correction technique disclosed in JPA63-207670 and JPA2000-335010 to the color image forming apparatus, the conveyance path of recording medium becomes very long. Thus, the conveyance misalignment occurs between a control position and an image transfer position and conveyance cannot be accurately conducted. Further, there is a problem that the size of the apparatus becomes large.
In the high quality image formation required in a POD (Print On Demand) market, a screen process is conducted. In this case, if the correction against the misalignment of a sheet inclination is conducted by adjusting the angle of the image, moiré occurs due to the relation between an adjusting angle and a screen angle. As a result, there is a problem that image quality becomes worse.
Since there is a problem when utilizing the techniques disclosed in JPA63-207670 and JPA08-268610 in the high-speed color image forming apparatus as described above, the technique for adjusting the misalignment by utilizing a loop or the techniques of misalignment adjustment, namely a techniques for adjusting the misalignment by shifting the conveyance roller in the shaft direction of the roller or revolving the shaft direction, which have been disclosed in JPA2000-335010 and JPA2005-35709 need to be used. However, it is difficult to adjust misalignment with high accuracy required in the POD field by applying these adjusting techniques. Image position misalignment, namely the misalignment of the image position from the designated position, which is positioned in a predetermined position against the sheet edge, becomes a problem.
In the case of the printer used in an office, where character images are mainly used, the misalignment in the degree of 1-2 mm will be acceptable. However, in the POD field, taking into account of imposition cutting and folio position, it is necessary to suppress the image position misalignment not more than 0.5 mm.
In the case when conducting misalignment adjustment by using a loop, which has been widely utilized, or conducting misalignment adjustment based on the shift of the conveyance roller in the shaft direction and revolution of the conveyance roller as disclosed in JPA2000-335010 and JPA2005-35709, the misalignment not less than 0.5 mm still resides and it is difficult to obtain satisfied quality. In order to satisfy the conditions previously described, extremely high parts accuracy is required and at the same time, durability will be lowered.
An object of the present invention is to form an image with high image position accuracy at the degree required by the POD field and to provide an image forming apparatus, which is capable of printing a high quality image.